The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the measurement and quanitification of ghost artifacts in MR images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The quality of a reconstructed MR image depends on many factors. One of these factors is the phase of the acquired NMR signal. In the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp", for example, the phase and frequency of the NMR signal produced by spin magnetization determines the location of the spins in the reconstructed image. As discussed in an article entitled "Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980), the Fourier transform method of imaging employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a "frequency encoding" direction orthogonal to the phase encoding direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
If the NMR measurements are not accurately made and either the phase or frequency of the acquired NMR signals is affected, so-called ghost artifacts will appear in the phase encoding direction and produce bright areas in the reconstructed image to either side of their true location. Similarly, frequency errors misplace spin magnetization in the frequency encoding direction and produce one or more ghost artifacts displaced from their true location.
Ghost artifacts can be caused by many factors and the level of ghost artifacts in an MR image is one measure of the operation of the MRI system. Unfortunately, there is no objective measure of the amount of ghosting, and hence, no quantitative measure of the performance of the MRI system.